Metal finishing Q&As since 1989

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Al Cast plating for high humidity and drinking water contact.

We are designing a few aluminum alloy die cast parts that will go in a consumer produt that will see steam and drinking water intermittently. What type of plating would hold up the best that also is food contact safe? Appearance of finish is not important since parts are not visible to consumer. I am concerned about long term use with chrome.

Juan Escobart
- New York City, NY USA
 

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I would probably consider tin or electroless nickel.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

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Anodizing or hard anodizing can add corrosion resistance to aluminum die-castings. Anodize is widely used in food contact applications. On die-castings, anodic coatings are a dirty gray color and not pretty to look at. But for this application they may work.

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.

Wadsworth, Ohio

 

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For potable water use, tin plating is hard to beat & is widely used for this purpose. Both tin and tin oxide, the native oxide that forms on the surface, have super low solubility constants in water, similar to the solubility of glass. Tin plate has a long history on potable water contact surfaces & it can be made to comply with NSF standards for full contact water. If a water container is made of metal & it is not stainless, it is a good bet it is tin plated "something".

As long as the pH of the water is close to 7, chrome plating or nickel would be fine, and some light anodizing would probably do the job (if the anodizing is not light, the roughened surface might encourage microbiotic growth). Actually, if the pH of the contact water is always near neutral 7, the bare aluminum is perfectly acceptable. Aluminum is amphoteric & becomes more soluble as the pH of the water becomes more acidic or more basic.

Dale Woika
Surface Conversion Sciences - Bellefonte, PA, US